Aluminum alkyls and especially triethylaluminum are used in the so-called Ziegler chain growth process to prepare linear alpha-olefins. The process involves the reaction of triethylaluminum (TEA) and ethylene at temperatures in the range of 200.degree.-500.degree. F. and pressures in the range of 2000 to 5000 psig to yield a mixture of tri-C.sub.2 -C.sub.20+ alkylaluminum compounds and C.sub.2 -C.sub.20+ olefins. Linear alpha-olefins are then recovered from the alkylaluminum compounds by olefin displacement using ethylene, 1-butene or mixtures thereof as the displacing olefin. Prior to displacement, the ethylene is flashed from the chain growth product and some of the alpha-olefins such as C.sub.4 -C.sub.6 and C.sub.8 -C.sub.12 cuts are removed by distillation. Because of the similar boiling points of TEA and 1-dodecene, the C.sub.8 -C.sub.12 cut is contaminated with TEA.
The TEA can be removed by hydrolysis but this causes a loss of the expensive TEA starting material. Accordingly, the amount of C.sub.8 -C.sub.12 olefins which can be economically removed from the process is limited so as to minimize the loss of TEA. The remaining C.sub.8 -C.sub.12 must be recycled thought the process. U.S. Pat. No. 3,149,179 discloses the use of membrane permeation to separate TEA from linear olefins. Preferred members are those selected from synthetically derived plastics which are predominantly hydrocarbon in nature and especially polyethylene. Polyesters are also mentioned. No working example is provided and the process is described in mostly general terms. In fact, we have found that a successful membrane separation of aluminum alkyls and alpha-olefins is difficult to achieve even considering the advances in membrane technology which have occurred in the approximately 30 years since the issuance of the U.S. Pat. No. 3,149,179 . We tried membranes based on polysulfones, polyvinyl alcohol, and ethyl acetate unsuccessfully. We have now discovered a process which, by using a polyphenylene oxide-derived membrane at elevated pressures, successfully separates aluminum alkyls from alpha-olefins.